huangyhg

cmu wall cracks
about 2 years i designed a one-story 16ft tall middle school addition with 8" load-bearing walls and roof composed of vulcraft joists (@6 ft on center) spanning between 8' and 20' + 1.5" metal roof deck . the cmu walls are reinforced with #5@32" and #5 rebar at window and door jambs and headers. all the headers have bond beams with 2#5's. also, control joints are located at 40-ft o.c.
no additional rebar was called out at the window sills.
having said this, hairline cracks are developing in the cmu walls at several of the window sill locations, and at the headers of some of the double doors. we have total of 4 cracks on this 30,000-sf addition (about 3,000 lf of walls), but they occur in some of the most visible locations.
the owner is concerned that these cracks could somehow compromise the intergrity of the building.
question: how do i go about re-assuring the owner that the building is safe and the cracks are cosmetic, without seeming dimissive??
check out our whitepaper library.
but i am curious how wide the door and window is. is there a parapet and are all cells solid grouted?
you probably can show him your calculation or let him to see other buildings you designed that similar to this one.

when you say headers are bond beams, do you mean the bars are continuous at that level or are they just lintels, locally over the window?
did you provide any joint reinforcement or other horizontal steel?
either one of those systems should provide adequate horizontal steel.
if you have adequate horizontal steel and steel around the openings per aci 530, section 1.13, you've followed the code and the cracks shouldn't be critical.
not a structural problem, but the windows or openings are a prime source for cracking to occur due to the reduced wall area and the square corners created by the reduced wall heights.
were any control joints designed near the openings to control the cracking and provide proper sealing from the elements?
the national concrete masonry association (ncma)tek notes include valuable guidelines that you could use to explain the potential cosmetically objectionable cracks. they can be found at the ncma site (ncma.org). just look to the right upper corner on the home page and click on the tek note, then select a state (any state) and a producer
look closely on the cracks see if there is indication of water penetration. i have had bad experience with such problems at home.
what sort of climate? i typically put joints in at 30', but 40' is not excessive. what sort of horizontal reinforcing? i typically use 9ga at 16" o/c vertically for load bearing and exterior and 9ga at 24" o/c for others. can the cracks be routed out and repaired? repair often makes a visual problem worse. do you have joints in the vicinity of the window/door openings? how long are the walls? what was the weather at the time of construction?
dik
is the wall constructed with stack bond or running bond? is there any joint reinforcing or regularly spaced bond beams?

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